Induction melting systems gain popularity as the most environmentally clean and reasonably efficient method of melting metal. In the induction melting furnace 1 shown in FIG. 1, the electromagnetic field produced by AC current in coil 2 surrounding a crucible 3 couples with metal or other conductive materials 4 inside the crucible and induces eddy currents 5, which in turn heat the metal. As indicated in FIG. 1, the arrows associated with coil 2 generally represent the direction of current flow in the coil, whereas the arrows associated with eddy currents 5 generally indicate the opposing direction of induced current flow in the conductive materials. Variable high frequency ac (typically in the range from 100 to 10,000 Hz) current is generated in a power supply or in a power converter 6 and supplied to coil 2. The converter 6, typically but not necessarily, consists of an AC-to-DC rectifier 7, a DC-to-AC inverter 8, and a set of capacitors 9, which, together with the induction coil, form a resonant loop. Other forms of power supplies, including motors-generators, pulse-width modulated (PWM) inverters, and the like, can be used.
As shown in FIG. 2, the magnetic field causes load current 10 to flow on the outside cylindrical surface of the conductive material, and coil current 11 to flow on the inner surface of the coil conductor. Crucible 3 in a typical furnace is made from ceramic material and usually is not electrically conductive. The efficiency of the furnace is computed by the formula:                     η        =                  1                      1            +                                                            D                  1                                                  D                  2                                            ·                                                ρ                  1                                                  ρ                  2                                            ·                                                Δ                  2                                                  Δ                  1                                                                                        equation        ⁢                                  ⁢                  (          1          )                    
where
η=furnace efficiency;
D1=coil inner diameter;
D2=load outer diameter;
ρ1=resistivity of coil winding material (copper);
ρ2=resistivity of load (melt);
Δ1=current depth of penetration in copper winding; and
Δ2=current depth of penetration in load (melt).
The depth of current penetration (Δ) is a function of a material's properties as determined by the formula:                     Δ        =                  k          ·                                    ρ                              f                ·                μ                                                                        equation        ⁢                                  ⁢                  (          2          )                    
where:
ρ=resistivity in ohm·meters;
f=frequency in Hertz;
μ=magnetic permeability (dimensionless relative value); and
Δ=depth of penetration in meters.
The constant, k=503, in equation (2) is dimensionless.
Because current does not penetrate deep into the low resistivity copper material of the coil, the typical coil efficiency is about 80 percent when the molten material is iron. Furnaces melting low resistivity materials such as aluminum (with a typical resistivity value of 2.6×10−8 ohm·meters), magnesium or copper alloys have a lower efficiency of about 65 percent. Because of significant heating due to electrical losses, the induction coil is water-cooled. That is, the coil is made of copper tubes 12 and a water-based coolant is passed through these tubes. The presence of water represents an additional danger when melting aluminum, magnesium or their alloys. In case of crucible rupture, water may combine with molten aluminum and a violent chemical reaction may take place in which the aluminum combines with oxygen in the water, releasing free hydrogen which may cause an explosion. Contact between water and magnesium may similarly result in an explosion and fire. Extreme caution is taken when aluminum or magnesium is melted in conventional water-cooled furnaces.
An object of the present invention is to improve the efficiency of an induction furnace by increasing the resistance of the load by using as the load a crucible made of a high temperature electrically conductive material or a high temperature material with high magnetic permeability. It is another object of the present invention to improve the efficiency of an induction furnace by reducing the resistance of the induction coil by using as the coil a cable wound of multiple copper conductors that are isolated from each other. It is still another object of the invention to properly select operating frequencies to yield optimum efficiency of an induction furnace.
It is a further object of the present invention to provide a high efficiency induction melting system with a furnace and power supply that do not use water-cooling and can be efficiently air-cooled.